1. Field of the Invention
This invention relates to a semiconductor device and, more particularly, to a semiconductor device having a two-layer gate structure having a floating gate electrode and a control gate electrode.
2. Description of the Related Art
A semiconductor device having a two-layer gate structure having a floating gate electrode and a control gate electrode, e.g., EPROM, has a structure as shown in FIGS. 1A and 1B.
FIGS. 1A and 1B are sectional views of a semiconductor device, taken perpendicular to each other. This device comprises P-type semiconductor substrate 101, which has N-type source and drain regions 102 and 103 formed in surface regions thereof. Gate insulating film 106 is formed on channel region 104, which extends between source and drain regions 102 and 103, and as is shown in FIG. 1B, channel region 104 is isolated by field insulating layer 105. Floating gate electrode 107, consisting of polycrystalline silicon, for example, is formed on gate insulating film 106, and silicon oxide (SiO.sub.2) film 108 is formed on electrode 107, by thermal oxidation thereof. Silicon nitride (Si.sub.3 N.sub.4) film 109 is formed on silicon oxide film 108, with silicon oxide film 110, in turn, being formed on film 109. Control gate electrode 111, consisting of polycrystalline silicon, is formed over floating gate electrode 107 via the three-layer insulating film consisting of silicon oxide film 108, silicon nitride film 109, and silicon oxide film 110.
The entire surface of the system, inclusive of control gate electrode 11, is then covered by insulating layer 112; for example, a silicon oxide film. In addition, although not shown, contact holes and aluminum leads are provided.
In the semiconductor device of the above structure, floating gate electrode 107 is in an electrically floating state. Therefore, when a high voltage is applied to control gate electrode 111, an electric field is generated in gate insulating film 106 due to coupling between control and floating gate electrodes 111 and 107 and coupling between floating gate electrode 107 and channel region 104. At the same time, by applying a high voltage to drain region 103 hot electrons are generated in channel region 104 near the drain region. These hot electrons are injected into floating gate electrode 107 to obtain a state in which data stored.
In this state, a high electric field is generated between control and floating gate electrodes 111 and 107. This means that an insulating film having a high breakdown voltage is required. For reducing the device size, on the other hand, reduction of the thickness of the insulating film is required.
In the semiconductor device noted above, the insulating film between control and floating gate electrodes 111 and 107 has a three-layer structure consisting of silicon oxide film 108, silicon nitride film 109 and silicon oxide film 110. The three-layer insulating film, compared to a single-layer insulating film, e.g., a silicon oxide film, has superior breakdown voltage for the same film thickness, so that it is advantageous over the single-layer film for thickness reduction. Even with the three-layer insulating film consisting of silicon oxide film 108, silicon nitride film 109 and silicon oxide film 110, there is a limitation imposed on the lower limit of the film thickness. More specifically, if the film thickness of silicon oxide layers 108 and 110 is less than 30 to 40 angstroms, a phenomenon of tunneling occurs, i.e., holes penetrate silicon oxide films 108 and 110. For this reason, silicon oxide films 108 and 110 should have a thickness of at least 40 angstroms. Further, if silicon nitride film 109 has an insufficient thickness, floating gate electrode 107 extending beneath film 109 is oxidized when forming silicon oxide film 110 by oxidation. For this reason, silicon nitride film 109 should have a thickness of at least 60 to 80 angstroms. Therefore, the three-layer insulating film should inevitably have a thickness of 140 to 160 angstroms.
In a further aspect, silicon nitride film 109, although excellent in regard to breakdown voltage, is very liable to capture electrons. Therefore, a failure in the erasing of data by ultraviolet ray illumination sometimes occurs, leading to deterioration of the erasing characteristics.